Paleo- mesoproterozoic deformation recorded within the Five Points shear zone, Wet Mountains, central Colorado.

摘要

Crustal growth during the Paleoproterozoic (2.5-1.6 Ga) consisted mostly of repeated accretion of ocean island arcs to the Archean Wyoming craton, concluding with the Mazatzal orogeny which ended by ∼1.63 Ga. Continental accretion during this period is bounded to the north by the Cheyenne belt in southern Wyoming. However, recently deformation and metamorphism have been dated south of the Cheyenne belt in southern Wyoming and northern Colorado between ∼1.62-1.58 Ga. Such events require adjustment of existing models for Paleoproterozoic crustal growth to indicate that either intracratonic tectonism occurred greater than 1000 km north of the southern margin of Laurentia, or that crustal assembly was not completed until more than 50 m.y. after the end of the Mazatzal orogeny as currently defined.;Two hypotheses have been proposed to explain 1.6 Ga deformation in southern Wyoming and northern Colorado: (1) intracratonic deformation more than 1000 km inboard of the continental margin that was similar in style to the Cenozoic Laramide orogeny, and would imply continental stability by ∼1.6 Ga. (2) ca. 1.6 Ga deformation represents "delayed" subduction of a north-dipping, 150-200 km-long Cheyenne slab in southern Wyoming and represents late stage crustal assembly. The goal of this project is to obtain data that will support or refute one or more of these models. This research will give a better understanding of the nature of assembly of the North American continent during the Paleoproterozoic, the tectonic processes acting on these continental blocks at approximately 1.6 Ga, as well as possibly leading to reevaluation of existing models.;Deformation recorded in Proterozoic rocks of Colorado in both the Front Range and the western Rocky Mountains, make the Five Points shear zone field area a perfect location to investigate the processes that formed the crust during this time. This was done through field mapping, using kinematic indicators, orientations of strain signatures, as well as by establishing pressure-temperature conditions of deformation within the shear zones and neighboring blocks using quantitative thermobarometry, isochemical (pseudosection) analysis, and determination of the age of deformation using a variety of geochronometers. Fault rocks such as mylonites and cataclasites typically contain datable syn-kinematic minerals, some of which form excellent kinematic indicators. Previous work in this region suggests widespread deformation occurred throughout Colorado ca. 1.6 Ga and will serve as a basis for this work.